Generally, a multi-stage sprocket comprising two or more sprockets with a different number of teeth is mounted to a rear hub of the bicycle so that a driving chain is moved axially of the sprocket by means of a derailleur and switched to a desired sprocket for transmitting the driving force to a rear wheel in a given speed change ratio.
The sprocket is formed of a steel plate punched by a punching mold. The portion of the mold corresponding to the crest of each tooth at the sprocket is formed in a circular arc with respect to the axis of rotation of sprocket, whereby the tooth crest of each tooth at the punched sprocket, as shown in FIG. 10, is convex with respect to the phantom plane connecting both circumferential edges C of the tooth along the addendum circle.
The driving chain, when switched from the smaller diameter sprocket to the larger diameter one at the sprocket assembly, is caught and raised by the edge C of the tooth at the larger diameter sprocket through the edge of an outer link plate or the end of a pin projecting therefrom at the chain brought into press contact with the lateral side of tooth at the larger diameter sprocket. When this occurs, a slip is generated between the convex tooth crest of the tooth at the larger diameter sprocket and the driving chain, so that a catch of the chain is inaccurate to delay engagement of the chain with the sprocket to that extent, resulting in a poor speed change efficiency. On the other hand, the chain, when switched from the larger diameter sprocket to the smaller diameter one, is raised to once ride on the tooth crest at the larger diameter sprocket and then engages with the teeth at the smaller diameter sprocket. When this occurs the chain riding on the convex tooth crest is not stable and may slip down therefrom.
Friction between the tooth crest and the chain riding thereon will cause considerable wear at both the circumferential edges. As a result, the curvature of the edge becomes larger, in other words, the roundness thereof becomes blunt, to reduce the convature of the convex tooth crest as a whole. Hence, the catch of the chain by the edge worsens and the chain riding on the tooth crest becomes further unstable.